How to write log to file

Question

I\'m trying to write to a log file with Go.

I have tried several approaches, all of which have failed. This is what I have tried:

func TestLogging(t          


        

Answer 1

os.Open() must have worked differently in the past, but this works for me:

f, err := os.OpenFile("testlogfile", os.O_RDWR | os.O_CREATE | os.O_APPEND, 0666)
if err != nil {
    log.Fatalf("error opening file: %v", err)
}
defer f.Close()

log.SetOutput(f)
log.Println("This is a test log entry")

Based on the Go docs, os.Open() can't work for log.SetOutput, because it opens the file "for reading:"

func Open

func Open(name string) (file *File, err error) Open opens the named file for reading. If successful, methods on the returned file can be used for reading; the associated file descriptor has mode O_RDONLY. If there is an error, it will be of type *PathError.

EDIT

Moved defer f.Close() to after if err != nil check

Answer 2

Building on Allison and Deepak's answer, I started using logrus and really like it:

var log = logrus.New()

func init() {

    // log to console and file
    f, err := os.OpenFile("crawler.log", os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666)
    if err != nil {
        log.Fatalf("error opening file: %v", err)
    }
    wrt := io.MultiWriter(os.Stdout, f)

    log.SetOutput(wrt)
}

I have a defer f.Close() in the main function

Answer 3

Declare up top in your global var so all your processes can access if needed.

package main

import (
    "log"
    "os"
)
var (
    outfile, _ = os.Create("path/to/my.log") // update path for your needs
    l      = log.New(outfile, "", 0)
)

func main() {
    l.Println("hello, log!!!")
}

Answer 4

If you run binary on linux machine you could use shell script.

overwrite into a file

./binaryapp > binaryapp.log

append into a file

./binaryapp >> binaryapp.log

overwrite stderr into a file

./binaryapp &> binaryapp.error.log

append stderr into a file

./binaryapp &>> binalyapp.error.log

it can be more dynamic using shell script file.

Answer 5

I'm writing logs to the files, which are generate on daily basis (per day one log file is getting generated). This approach is working fine for me :

var (
    serverLogger *log.Logger
)

func init() {
    // set location of log file
    date := time.Now().Format("2006-01-02")
    var logpath = os.Getenv(constant.XDirectoryPath) + constant.LogFilePath + date + constant.LogFileExtension
    os.MkdirAll(os.Getenv(constant.XDirectoryPath)+constant.LogFilePath, os.ModePerm)
    flag.Parse()
    var file, err1 = os.OpenFile(logpath, os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666)

    if err1 != nil {
        panic(err1)
    }
    mw := io.MultiWriter(os.Stdout, file)
    serverLogger = log.New(mw, constant.Empty, log.LstdFlags)
    serverLogger.Println("LogFile : " + logpath)
}

// LogServer logs to server's log file
func LogServer(logLevel enum.LogLevel, message string) {
    _, file, no, ok := runtime.Caller(1)
    logLineData := "logger_server.go"
    if ok {
        file = shortenFilePath(file)
        logLineData = fmt.Sprintf(file + constant.ColonWithSpace + strconv.Itoa(no) + constant.HyphenWithSpace)
    }
    serverLogger.Println(logLineData + logLevel.String() + constant.HyphenWithSpace + message)
}

// ShortenFilePath Shortens file path to a/b/c/d.go tp d.go
func shortenFilePath(file string) string {
    short := file
    for i := len(file) - 1; i > 0; i-- {
        if file[i] == constant.ForwardSlash {
            short = file[i+1:]
            break
        }
    }
    file = short
    return file
}

"shortenFilePath()" method used to get the name of the file from full path of file. and "LogServer()" method is used to create a formatted log statement (contains : filename, line number, log level, error statement etc...)